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Impacts of Nonnative Brown Trout Salmo trutta on Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in a Tributary Stream
Abstract
Nonnative trout are a considerable threat to native salmonids yet our understanding of the mechanisms behind interspecific interactions remains limited. We evaluated the impacts of nonnative Brown Trout Salmo salar on a population of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in Montana, USA. We contrasted diets, growth, and survival of Yellowstone Cutthroat Trout occurring in allopatry (i.e., where no brown trout were present) with individuals sympatric (i.e., co‐occurring) with nonnative Brown Trout. We assessed summer and autumn diets using gastric lavage methods and survival and growth using mark‐recapture analyses. Overlap in diets at sites where Yellowstone Cutthroat Trout were sympatric with Brown Trout was high during July (Horn's index, H = 0.94) and October (H = 0.83). In the presence of Brown Trout, Yellowstone Cutthroat Trout growth rates were significantly lower for juvenile (<175 mm) length and adult (≥175 mm) length and mass than in allopatric sites. Allopatric Yellowstone Cutthroat Trout survival was greater across size classes with the most pronounced difference in the age‐2 size class (125‐175 mm). Together, these results in concert with observed changes in length‐frequency data, indicating a considerable lack of Yellowstone Cutthroat Trout recruitment where sympatric with Brown Trout, suggest the negative implications of Brown Trout are notable. This article is protected by copyright. All rights reserved.
... early maturity, faster growth) than stressful thermal conditions. However, Huntsman et al. (2021a) also observed apparent survival of RGCT was lower than expected in a cool stream with brown trout present, a pattern consistent with observations for other cutthroat trout subspecies in sympatry with brown trout (Al-Chokhachy & Sepulveda, 2019;Budy et al., 2007). ...
... subspecies(Al-Chokhachy & Sepulveda, 2019;Budy et al., 2007) and RGCT populations(Huntsman et al., 2021a).Density effects were as equally important as species interactions in affecting trout apparent survival rates in this study. Both allopatric RGCT and brown trout populations showed similar demographic responses to density effects, where apparent survival decreased with increasing densities. ...
... Even though McHugh and Budy (2006) did not observe a brown trout effect on cutthroat trout survival, they did find evidence of a brown trout effect on other life-history traits such as growth and foraging ecology, all of which could have indirect consequences on cutthroat trout survival over the longer term.We found a negative impact by brown trout on RGCT survival, but these effects may have been greater if size class was considered. Indeed, other studies have shown a greater susceptibility of juvenile cutthroat trout to non-native trout invasions(Al-Chokhachy & Sepulveda, 2019; Huntsman et al., 2021a;Peterson et al., 2004). classes were also hypothesised byMcHugh and Budy (2006) as a potential reason that a brown trout effect was not observed on Bonneville cutthroat trout survival, as they did not collect enough juvenile fish to test a size class (i.e. ...
Environmental stressors associated with a changing climate and non‐native fish, individually, represent significant threats to native fish conservation. These threats can exacerbate risks to native fishes when conditions interact at the trailing edge of a population's distribution. We collected capture–mark–recapture data for Rio Grande cutthroat trout (RGCT, Oncorhynchus clarkii virginalis) at the trailing edge of all cutthroat trout distributions from eight northern New Mexico populations. We used a factorial sampling design from streams characterised as “cool” or “warm” and whether RGCT were sympatric with non‐native brown trout (Salmo trutta). We tested competing hypotheses that warm temperatures, reduced flows, high densities and sympatry with brown trout would negatively impact RGCT apparent survival rates. We found the strongest evidence for a non‐native trout interaction with total trout density affecting RGCT apparent survival rates. Our results are consistent with patterns observed in northern cutthroat trout populations where non‐native salmonids negatively impacted apparent survival rates. We also found that a negative density effect was observed on allopatric RGCT and sympatric brown trout apparent survival, but a positive density effect was observed for sympatric RGCT. These results suggest higher density populations of RGCT may be more resilient to displacement by non‐native trout than low‐density populations.
... An iconic species already suffering tenfold reductions in range in the southern Rocky Mountains since European settlement (Penaluna et al. 2016;Budy et al. 2019), cutthroat trout have long been the focus of intense conservation efforts (Gresswell 1988;Trotter 2008;Penaluna et al. 2016). Declines are primarily driven by the invasion of nonnative trout (Peterson et al. 2004;Fausch 2008;Meredith et al. 2017;Al-Chokhachy and Sepulveda 2019;Zeigler et al. 2019), but the advent of a warming climate will bring additional challenges (Williams et al. 2009;Wenger et al. 2011;Isaak et al. 2012;Roberts et al. 2017). The distribution of trout is predicated by thermal requirements (Dunham et al. 2003;Al-Chokhachy et al. 2013;Isaak et al. 2017), and some scientists have predicted substantial range contractions as a result of increasing temperature (Williams et al. 2009;Wenger et al. 2011;Isaak et al. 2012;Eby et al. 2014). ...
With temperatures expected to rise across the southern Rocky Mountains, the ability of native fishes to tolerate stream warming has become a critical concern for those tasked with preserving coldwater species. We used common garden experiments to evaluate the thermal tolerance of cutthroat trout ( Oncorhynchus clarkii) fry from five populations important to managers representing three sub-species. Critical thermal maxima (CTMs) were evaluated through traditional exposure trials, while optimal growth and ultimate upper incipient lethal temperatures (UUILTs) were examined over the course of 21-day trials at six static temperature treatments. Whereas CTMs differed among populations (mean = 27.91 °C, SD = 0.35 °C), UUILTs did not (mean = 24.40 °C, SD = 0.04 °C). Comparison of cubic temperature-growth functions to the traditional quadratic functions showed that adding a third-order term for temperature can improve model fit, and revealed substantial differences in optimal growth temperatures (15.4–18.3 °C). Knowledge of these thermal tolerance thresholds will help to predict the consequences of a warming climate, identify suitable habitats for repatriation, and inform water quality temperature standards established to protect these fish into the future.
... Brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) are primary causes of decline in many native fishes in cool-temperate parts of the Southern Hemisphere (Young et al., 2010;Raadik, 2014) and around the world (McDowall, 2006;Kirk et al., 2018;Al-Chokhachy & Sepulveda, 2019). Both species are widespread and found across a broad altitudinal range from lowland environments to small headwater streams. ...
Find the report here: https://bookdown.org/hughallan/asfb-drone-report/. The original HTML file can be downloaded from here: https://drive.google.com/u/0/uc?id=1YZxH_kRd8nHCrbBHrrfhus0YGdBYwjGp&export=download
... Apart from the investigations of brown trout invasion on the riverine ecosystems, some rare, albeit classic studies have investigated the invasion-induced 'life history' responses of the native fish fauna (Näslund et al. 1998;McHugh and Budy 2005;Ö hlund et al. 2008;Hoxmeier and Dieterman 2012;Al-Chokhachy et al. 2016;Al-Chokhachy and Sepulveda 2019). Life-history traits like fecundity, egg diameter, size and age at maturity show major variation across the biotic and abiotic stressors (Stearns 1992), and are idiosyncratic to the environmental setup a species is placed in. ...
After 150 years of introduction of the brown trout Salmo trutta in Himalaya, the native species’ response to this globally pervasive invader, is still unknown. Here, we investigate the effects of invasion of brown trout on native snow trout Schizothorax richardsonii, one of the most primitive species that co-evolved with the Himalayan orogeny. We contrast two natural river systems which harbour snow trout in the absence (allopatry) and presence (sympatry) of brown trout. We put forth that sympatric snow trout adapted to a ‘fast’ life history with maturation at a smaller length, greater fecundity and smaller egg diameter to cope with brown trout invasion. However, investment in a reproductive-somatic trade off was evident with a disrupted size structure and reduced abundance as compared to the allopatric population. Although the fast life history adaptations of snow trout might increase their competitive ability with invasive brown trout, trading off the somatic fitness in the process seemingly acts as a deterrent to longevity. We attribute the plastic responses of snow trout to their plausible inherent potential of sustenance and recovery from high invasion pressures. The popularity of brown trout as a sport fish in Himalaya however poses extraneous propagule pressure on the snow trout, which warrants quantification through future research.
... The shaded area represents the 95% credible interval, the dark black line is the mean, and light grey lines are each individual posterior estimate of the demographic parameter. Transition probabilities are reported as monthly rates direct impact of brown trout on cutthroat trout survival has been variable, where cutthroat trout survival can be significantly lower in the presence of brown trout (Budy, Thiede, & McHugh, 2007), no strong brown trout effects on cutthroat trout survival may occur (McHugh & Budy, 2005), and stronger effects within immature than mature cutthroat trout states (Al-Chokhachy & Sepulveda, 2019) have all been documented. Although we have evidence for a brown trout effect on RGCT survival, a direct test of these effects would require a greater number of invaded streams be studied to confirm such an effect. ...
The impacts of climate change on cold‐water fishes will likely negatively manifest in populations at the trailing edge of their distributions. Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis, RGCT) occupy arid south‐western U.S. streams at the southern‐most edge of all cutthroat trout distributions, making RGCT particularly vulnerable to the anticipated warming and drying in this region. We hypothesised that RGCT possess a portfolio of life‐history traits that aid in their persistence within streams of varying temperature and stream drying conditions. We used otolith and multistate capture–mark–recapture data to determine how these environmental constraints influence life‐history trait expression (length‐ and age‐at‐maturity) and demography in RGCT populations from northern New Mexico, United States. We found evidence that RGCT reached maturity fastest at sites with warm stream temperatures and low densities. We did not find a strong relationship between discharge and any demographic rate, although apparent survival of mature RGCT decreased as stream temperature increased. Our study suggests plasticity in trait expression may be a life‐history characteristic which can assist trailing edge populations like RGCT persist in a changing climate.
... Brook trout have replaced native cutthroat trout in much of their historical range and continue to invade cutthroat trout habitats [83]. Brown trout are actively invading streams in western Montana and negatively affecting native bull trout [84] and Yellowstone cutthroat trout [85]. Brown trout are increasing in abundance and dispersing downstream in the Colorado River below Glen Canyon Dam, where they are threatening native fishes [86,87]. ...
Preventing the interbasin transfer of aquatic invasive species is a high priority for natural resource managers. Such transfers can be made by humans or can occur by dispersal through connected waterways. A natural surface water connection between the Atlantic and Pacific drainages in North America exists at Two Ocean Pass south of Yellowstone National Park. Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri used this route to cross the Continental Divide and colonize the Yellowstone River from ancestral sources in the Snake River following glacial recession 14,000 bp. Nonnative lake trout Salvelinus namaycush were stocked into lakes in the Snake River headwaters in 1890 and quickly dispersed downstream. Lake trout were discovered in Yellowstone Lake in 1994 and were assumed to have been illegally introduced. Recently, lake trout have demonstrated their ability to move widely through river systems and invade headwater lakes in Glacier National Park. Our objective was to determine if lake trout and other nonnative fish were present in the connected waters near Two Ocean Pass and could thereby colonize the Yellowstone River basin in the past or future. We used environmental DNA (eDNA), electrofishing, and angling to survey for lake trout and other fishes. Yellowstone cutthroat trout were detected at nearly all sites on both sides of the Continental Divide. Lake trout and invasive brook trout S. fontinalis were detected in Pacific Creek near its confluence with the Snake River. We conclude that invasive movements by lake trout from the Snake River over Two Ocean Pass may have resulted in their colonization of Yellowstone Lake. Moreover, Yellowstone Lake may be vulnerable to additional invasions because several other nonnative fish inhabit the upper Snake River. In the future, eDNA collected across smaller spatial intervals in Pacific Creek during flow conditions more conducive to lake trout movement may provide further insight into the extent of non-native fish invasions in this stream.
To mitigate westslope cutthroat trout (WCT; Oncorhynchus clarkii lewisi) declines, Montana Fish, Wildlife, & Parks carries out large scale restorations, including wild-origin stocking efforts from conservation hatcheries. Although hatcheries have made progress in limiting the effects of artificial selection on stocked populations, concerns remain that rearing practices inadvertently reduce the diversity propagated into the wild.
The objective of this research was to identify traits of WCT driving poor survival or reproduction in a hatchery, allowing managers to reduce artificial selection by screening for fish requiring alternative rearing. In Chapter 1, I repeatedly measured 18 behavior, morphology, and health traits from hatchery intake (July 2019) to spawn (June 2021). I identified traits with low within- relative to between-individual variation as traits likely to be indicative of specialization. As specialists tend to maximize performance under a narrow range of conditions, they may be vulnerable to artificial selection within hatcheries. In Chapter 2, I tested whether the specialized traits identified in Chapter 1, growth rate, or age at hatchery intake of individual WCT could predict survival or reproduction.
In Chapter 1, I identified nine specialized (relative condition, weighted health index, health index, body shape, energetic reserves, latency, and upper jaw, lower jaw, and anal fin residual length) traits. I hypothesized these traits would predict later survival or reproductive performance. In Chapter 2, I identified October 2019 daily growth rate in weight and every replicate length measurement after October 2019 to strongly predict total ovulated eggs and total number of hatch embryos produced by females. However, among individual variation in the median percent hatch embryos was not explained by maternal size. Male gamete quality and fertilization success was consistently high, and I found no biologically significant predictors of reproductive performance for males. I also found no predictors of survival for males or females.
Despite high total ovulated eggs and total hatch embryo success for females, variable female median percent hatch embryos suggests that quality of ovulated eggs is driving current limitations to WCT hatchery reproduction. I recommend further investigation into impacts of rearing stressors and post-ovulatory aging on female WCT and their reproductive performance.
Henrys Lake, Idaho, is a renowned trophy trout fishery that faces an uncertain future following the establishment of Utah Chub (UTC) Gila atraria. Utah Chubs were first documented in the lake in 1993 and have become abundant over the past two decades. Little is known about the ecology of UTC, but they typically have negative effects on salmonids in systems where they have been introduced. We sought to fill knowledge gaps in UTC ecology and provide insight on potential interactions with Yellowstone Cutthroat Trout (YCT) Oncorhynchus clarkii bouvieri. Ninety‐four YCT and 95 UTC were radio‐tagged in spring 2019 and 2020 to better understand potential interactions between YCT and UTC in Henrys Lake. Fish were located via mobile tracking and fixed receivers from June to December 2019 and 2020. In June of both years, YCT and UTC were concentrated in nearshore habitats. As water temperatures increased, UTC were documented in deeper water (mean ± SD; 3.6 ± 1.4 m) and YCT became more concentrated in areas with cold water (e.g., mouths of tributaries, in‐lake springs). In July and August, large congregations of UTC were observed. Yellowstone Cutthroat Trout were detected in tributaries from June to August, but no UTC were detected in the tributaries. By late fall (November–December), YCT were located along the shoreline and UTC were detected in the middle of the lake. Both YCT and UTC were observed in areas with dense vegetation. Macrophytes likely provided a food source for UTC and cover from predators for both species. Yellowstone Cutthroat Trout locations were negatively related to warm water temperatures, whereas UTC were positively associated with warm water temperatures. Results from this research fill knowledge gaps in UTC and YCT interactions as well as provide valuable insight on the ecology of UTC and adfluvial Cutthroat Trout populations. Furthermore, distribution patterns and habitat selectivity of YCT and UTC in Henrys Lake can be used to inform management decisions for fishery improvement and YCT conservation.
Background: Due to the rapidly changing climatic conditions, South Korea faces the grand challenge of exotic species. With the increasing human movement, the influx of alien species to novel regions is prevalent across the globe. The latest research suggests that it is easy to prevent the introduction and establishment of alien species rather than controlling their spread and eradication. Like other countries, the Korean Ministry of Environment released a list (in 2018) of 45 potential risky exotic fish species considered likely to be invasive candidate fish species if they ever succeed in entering the Korean aquatic ecosystems.
Results: The investigation into the invasion suitability traits showed that potential risky fish species could utilize those features in becoming invasive once they arrive in the Korean aquatic ecosystems. If the novel species establish viable populations, they are likely to incur higher economic costs, damage the native aquatic fauna and flora, and jeopardize the already perilled species. Furthermore, they can damage the installed infrastructure, decline overall abundance and biodiversity, and disturb the ecosystem services. Here we reviewed the list of fish species concerning their family, native origin, preferred aquatic biomes, main food items, current status in Korea, and potential threats to humans and the ecosystems. Data shows that most species are either already designated as invasive in the neighboring counties, including Japan, Vietnam, Thailand, and China, or originate from these countries. Such species have a higher climate match with the Korean territories.
Conclusions: Therefore, it is exceptionally essential to study their most critical features and take regulatory measures to restrict their entry. The incoming fish species must be screened before letting them in the country in the future. The regulatory authorities must highlight the threatening traits of such species and strictly monitor their entrance. Detailed research is required to explore the other species, especially targeting the neighboring countries fish biodiversity, having demonstrated invasive features and matching the Korean climate.
Brown trout Salmo trutta is a potent global invader and its establishments have progressively altered physiologies, life‐histories and niche‐availabilities for native fish species. River impoundments further escalate its invasion potential. The Himalayan rivers however, stay uncharted for the effects of brown trout interactions with the native fish fauna. Snow trout Schizothorax richardsonii a Himalayan cold‐water native, concerningly overlaps its range with brown trout. To understand its responses to invasion pressures, we investigated brown trout effects on the age and growth of snow trout populations in three rivers with varying levels of perturbation: (a) a dammed and (b) an undammed river with the invasive brown trout in comparison to (c) an undammed river without invasion pressures. We found sympatric snow trout in the undammed river to respond to brown trout invasion with fast life history responses, showing an early age‐at‐maturity (A50 = 1.2 years) and fast growth with a higher growth constant (K = 0.40 yr−1) and specific rate of linear growth across life. On the contrary, sympatric snow trout in the dammed river showed an explicitly slow life‐history by maturing at a higher age (A50 = 2.9 years) and a slow growth, with a lower growth constant (K = 0.26 yr−1) and specific linear growth rates. Our findings suggest that, the snow trout appear to present stronger response to brown trout invasions when the river is unaltered and free from hydropower operations and damming. Further research is strongly warranted from other high‐altitude Himalayan basins to delineate the variation in growth strategies exhibited by snow trout in sympatry with the invasives.
The invasion of freshwater ecosystems by non‐native species can constitute a significant threat to native species and ecosystem health. Non‐native trouts have long been stocked in areas where native trouts occur and have negatively impacted native trouts through predation, competition, and hybridization. This study encompassed two seasons of sampling efforts across two ecoregions of the western United States: The Great Basin in summer 2016 and the Yellowstone River Basin in summer 2017. We found significant dietary overlaps among native and non‐native trouts within the Great Basin and Yellowstone River Basin ecoregions. Three orders of invertebrates (Ephemeroptera, Trichoptera, and Diptera) composed the majority of stomach contents and were responsible for driving the observed patterns. Great Basin trout had higher body conditions (k), and non‐native Great Basin trout had higher gut fullness values than Yellowstone River Basin trout, indicating a possible limitation of food in the Yellowstone River Basin. Native fishes were the least abundant and had the lowest body condition in each ecoregion. These findings may indicate a negative impact on native trouts by non‐native trouts. We recommend additional monitoring of native and non‐native trout diets, regular invertebrate surveys to identify the availability of diet items, and reconsidering stocking efforts that can result in overlap of non‐native fishes with native cutthroat trout.
Compensatory growth—when individuals in poor condition grow rapidly to catch up to conspecifics—may be a mechanism that allows individuals to tolerate stressful environmental conditions, both abiotic and biotic. This phenomenon has been documented fairly widely in laboratory and field experiments, but evidence for compensatory growth in the wild is scarce.
Cutthroat trout ( Oncorhynchus clarkii subsp.) are cold‐water specialists that inhabit montane streams in western North America where seasonal conditions can be harsh and growth rates vary greatly among seasons. Understanding if individuals compensate for periods of reduced growth and body condition will improve understanding of the requirements of fish throughout their life‐cycle and across freshwater habitats.
We quantified compensatory growth of juvenile cutthroat trout using extensive mark–recapture data from 11 stream populations (1,125 individuals) and two subspecies inhabiting a wide range of ecological settings in the northern Rocky Mountains, U.S.A . Our objectives were to determine how growth was linked across seasons and whether individuals behaviourally compensated for depressed body condition via emigration.
Fish in relatively poor condition consistently demonstrated compensatory growth in mass during subsequent seasons. In contrast, fish in relatively better condition responded with positive growth in length during the summer signalling these fish may be better suited to headwater environments; no compensatory growth in length was found during the winter. Furthermore, there was no evidence that individual condition mediated migration tendencies of fish to seek more favourable habitat.
Across a wide range of environmental conditions, we found consistent empirical support for compensatory growth in mass in the wild. A critical next step is to quantify how changing abiotic and biotic conditions influence the ability of stream fishes to compensate for locally or seasonally challenging conditions, thereby affecting long‐term resiliency, viability, and adaptation in the face of changing environmental conditions.
Many freshwater fish populations have been greatly reduced, with particular loss of migratory fishes. Recovering depleted populations is challenging as threats are often plentiful and complex, especially in arid environments where demands for water resources are high. Here, we describe how a collaborative, multifaceted approach has spurred natural reproduction—a major step towards Lahontan Cutthroat Trout Oncorhynchus clarkii henshawi (LCT) recovery in Pyramid Lake and the Truckee River, Nevada, once home to one of largest freshwater salmonids in North America. The factors limiting LCT were immense, including habitat fragmentation, degradation, and non‐native species‐attributes common in the declines of native salmonids. Yet for the first time in over 80 years and each year since 2014, adfluvial LCT have spawned in the lower Truckee River, resulting in the production of tens of thousands of young‐of‐year. The progress and positive trajectory towards recovery were driven by a holistic view of the Truckee River watershed beginning in the early 1990's that envisioned bringing numerous conservation building blocks together to expedite the conservation and recovery for the listed fishes of Pyramid Lake. Although additional challenges remain, the LCT recovery program in the Truckee River basin provides a template for the conservation of imperiled fishes. This article is protected by copyright. All rights reserved.
Demographic data are sparse for Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri; YCT). Data for YCT in the spawning run (spring; 29 years) of a Yellowstone Lake tributary or caught in gill nets set (fall; 30 years) at established lake locations between 1977 and 2007 were examined. Female proportion in runs averaged 0.61 but was 0.48 among gillnetted "prespawner" YCT (i.e., mature fish whose excised gonads indicated the fish would have spawned the next year). Maturity proportion-total length (TL) relationships for gillnetted female and male YCT were logistic-shaped and similar in their inflection points; maturity onset occurred at 200-250 mm TL; ~95% of YCT > 400 mm TL were mature and ~70% were prespawners. Fecundity was positively associated with YCT TL. Accuracy of scale-based YCT ages was affected by a frequently overlooked scale annulus and an inability to unequivocally identify fish of a single cohort on the basis of scale characteristics using associated, recognized ageing criteria for this population. Temporal differences in fits of a modified von Bertalanffy growth model to YCT TL at capture and scale-based age probably resulted from ageing errors evident among successive, annual scale analysts rather than differences in YCT growth. Nevertheless, when the age estimates of one, long-term analyst were used in analyses, the estimated growth parameters L and were concordant with empirical observations of the maximum TL of YCT and TL of age-1 YCT in Yellowstone Lake, respectively. The demographic relationships and linking, parameterized growth model provide a useful foundation for age-structured population modeling.
Brown trout are one of the most pervasive and successful invaders worldwide and are ubiquitous across the Intermountain West, USA (IMW). This species is the foundation of extremely popular and economically significant sport fisheries despite well-established negative effects on native fishes and ecosystems, resulting in very challenging, and often opposing, conservation and management goals. Herein, we review the direct (e.g., competition and predation) and indirect (e.g., disease vectors) pathways through which brown trout across the IMW have posed a threat to native species. We discuss the importance of brown trout as economically and culturally important fisheries, especially in novel tailwater ecosystems created by damming. To this end, we surveyed 24 experts from eight states across the IMW to document the relevance of novel brown trout fisheries in 51 tailwaters and found brown trout are thriving in these novel ecosystems, which are often unsuitable for native fishes. We discuss the challenging interplay between protecting native species and managing novel brown trout fisheries. Notably, the future of exotic brown trout in the IMW is shifting as the prestige of native fisheries is growing and many non-native eradication efforts have occurred. The future of exotic brown trout in the IMW, will depend on the nexus of public sentiment and policy, the effectiveness of eradication efforts, and the effect of climate change on both the native fishes and exotic brown trout. Regardless, because brown trout are pervasive and have a broad distribution through the IMW, populations of this species will likely persist at least in some locations into the future.
Hybridization between invasive and native species, a significant threat to worldwide biodiversity, is predicted to increase due to climate-induced expansions of invasive species. Long-term research and monitoring are crucial for understanding the ecological and evolutionary processes that modulate the effects of invasive species. Using a large, multidecade genetics dataset (N?=?582 sites, 12,878 individuals) with high-resolution climate predictions and extensive stocking records, we evaluate the spatiotemporal dynamics of hybridization between native cutthroat trout and invasive rainbow trout, the world's most widely introduced invasive fish, across the Northern Rocky Mountains of the United States. Historical effects of stocking and contemporary patterns of climatic variation were strongly related to the spread of hybridization across space and time. The probability of occurrence, extent of, and temporal changes in hybridization increased at sites in close proximity to historical stocking locations with greater rainbow trout propagule pressure, warmer water temperatures, and lower spring precipitation. Although locations with warmer water temperatures were more prone to hybridization, cold sites were not protected from invasion; 58% of hybridized sites had cold mean summer water temperatures (<11?C). Despite cessation of stocking over 40?years ago, hybridization increased over time at half (50%) of the locations with long-term data, the vast majority of which (74%) were initially nonhybridized, emphasizing the chronic, negative impacts of human-mediated hybridization. These results show that effects of climate change on biodiversity must be analyzed in the context of historical human impacts that set ecological and evolutionary trajectories.
The distribution of native brook trout (Salvelinus fontinalis) in eastern North America is often limited by temperature and introduced brown trout (Salmo trutta), the relative importance of which is poorly understood but critical for conservation and restoration planning. We evaluated effects of brown trout on brook trout behavior and habitat use in experimental streams across increasing temperatures (14–23 °C) with simulated groundwater upwelling zones providing thermal refugia (6–9 °C below ambient temperatures). Allopatric and sympatric trout populations increased their use of upwelling zones as ambient temperatures increased, demonstrating the importance of groundwater as thermal refugia in warming streams. Allopatric brook trout showed greater movement rates and more even spatial distributions within streams than sympatric brook trout, suggesting interference competition by brown trout for access to forage habitats located outside thermal refugia. Our results indicate that removal of introduced brown trout may facilitate native brook trout expansion and population viability in downstream reaches depending in part on the spatial configuration of groundwater upwelling zones.
Purposeful species introductions offer opportunities to inform our understanding of both invasion success and conservation hurdles. We evaluated factors determining the energetic limitations of brown trout (Salmo trutta) in both their native and introduced ranges. Our focus was on brown trout because they are nearly globally distributed, considered one of the world's worst invaders, yet imperiled in much of their native habitat. We synthesized and compared data describing temperature regime, diet, growth, and maximum body size across multiple spatial and temporal scales, from country (both exotic and native habitats) and major geographic area (MGA) to rivers and years within MGA. Using these data as inputs, we next used bioenergetic efficiency (BioEff), a relative scalar representing a realized percentage of maximum possible consumption (0?100%) as our primary response variable and a multi-scale, nested, mixed statistical model (GLIMMIX) to evaluate variation among and within spatial scales and as a function of density and elevation. MGA and year (the residual) explained the greatest proportion of variance in BioEff. Temperature varied widely among MGA and was a strong driver of variation in BioEff. We observed surprisingly little variation in the diet of brown trout, except the overwhelming influence of the switch to piscivory observed only in exotic MGA. We observed only a weak signal of density-dependent effects on BioEff; however, BioEff remained <50% at densities >2.5 fish/m2. The trajectory of BioEff across the life span of the fish elucidated the substantial variation in performance among MGAs; the maximum body size attained by brown trout was consistently below 400 mm in native habitat but reached ?600 mm outside their native range, where brown trout grew rapidly, feeding in part on naive prey fishes. The integrative, physiological approach, in combination with the intercontinental and comparative nature of our study, allowed us to overcome challenges associated with context-dependent variation in determining invasion success. Overall our results indicate ?growth plasticity across the life span? was important for facilitating invasion, and should be added to lists of factors characterizing successful invaders.
Although laboratory studies have provided evidence for negative interactions between brook trout and brown trout, it is unknown how these interactions affect larger scale demographics in a natural setting. We tested the effects of invasive brown trout on brook trout demographics by removing brown trout from a sympatric population using a before–after control-impact study design. The study was conducted across a large stream network for a period of 6 years. Abundance of brook trout increased after brown trout removal primarily as a result of increased recruitment and immigration. Size structure also shifted towards larger individuals as a result of increased growth rates and a decrease in emigration of larger trout. Size at maturity and body condition did not change after brown trout removal. Adult brook trout survival increased during the post-treatment period in both the treatment and control reach. A decrease in flood intensity during the post-treatment time period may have led to increased survival. Adult survival may not be the best metric to use when assessing interactions between trout species, especially when the subordinate species has suitable areas to emigrate.
Restoration of altered or degraded habitats is often a key component in the conservation plan of native aquatic species, but introduced species may influence the response of the native community to restoration. Recent habitat restoration of the middle section of the Provo River in central Utah, USA, provided an opportunity to evaluate the effect of habitat restoration on the native fish community in a system with an introduced, dominant predator-brown trout (Salmo trutta). To determine the change in distribution of fish species and community composition, we surveyed 200 m of each of the four study reaches both before restoration (1998) and after restoration (2007 and 2009). Juveniles and adults of six native species increased in distribution after restoration. The variation in fish community structure among reaches was lower post-restoration than pre-restoration. Overall, restoration of complex habitat in the middle Provo River led to increased pattern of coexistence between native fishes and introduced brown trout, but restoration activities did not improve the status of the river's two rarest native fish species. Habitat restoration may only be completely successful in terms of restoring native communities when the abundance of invasive species can be kept at low levels.
Understanding how climate change may facilitate species turnover is an important step in identifying potential conservation strategies. We used data from 33 sites in western Montana to quantify climate associations with native bull trout (Salvelinus confluentus) and non-native brown trout (Salmo trutta) abundance and population growth rates (λ). We estimated λ using exponential growth state-space models and delineated study sites based on bull trout use for either spawning and rearing (SR) or foraging, migrating, and overwintering (FMO) habitat. Bull trout abundance was negatively associated with mean August stream temperatures within SR habitat (r = −0.75). Brown trout abundance was generally highest at temperatures between 12 and 14 °C. We found bull trout λ were generally stable at sites with mean August temperature below 10 °C but significantly decreasing, rare, or extirpated at 58% of the sites with temperatures exceeding 10 °C. Brown trout λ were highest in SR and sites with temperatures exceeding 12 °C. Declining bull trout λ at sites where brown trout were absent suggest brown trout are likely replacing bull trout in a warming climate.
The distribution and future fate of ectothermic organisms in a warming world will be dictated by thermalscapes across landscapes. That is particularly true for stream fishes and cold-water species like trout, salmon, and char that are already constrained to high elevations and latitudes. The extreme climates in those environments also preclude invasions by most non-native species, so identifying especially cold habitats capable of absorbing future climate change while still supporting native populations would highlight important refugia. By coupling crowd-sourced biological datasets with high-resolution stream temperature scenarios, we delineate network refugia across >250 000 stream km in the Northern Rocky Mountains for two native salmonids-bull trout (BT) and cutthroat trout (CT). Under both moderate and extreme climate change scenarios, refugia with high probabilities of trout population occupancy (>0.9) were predicted to exist (33-68 BT refugia; 917-1425 CT refugia). Most refugia are on public lands (>90%) where few currently have protected status in National Parks or Wilderness Areas (<15%). Forecasts of refuge locations could enable protection of key watersheds and provide a foundation for climate smart planning of conservation networks. Using cold water as a 'climate shield' is generalizable to other species and geographic areas because it has a strong physiological basis, relies on nationally available geospatial data, and mines existing biological datasets. Importantly, the approach creates a framework to integrate data contributed by many individuals and resource agencies, and a process that strengthens the collaborative and social networks needed to preserve many cold-water fish populations through the 21st century.
Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
Hybridization between native and non-native species has serious biological consequences, but our understanding of how dispersal and selection interact to influence invasive hybridization is limited. Here, we document the spread of genetic introgression between a native (Oncorhynchus clarkii) and invasive (Oncorhynchus mykiss) trout, and identify the mechanisms influencing genetic admixture. In two populations inhabiting contrasting environments, non-native admixture increased rapidly from 1984 to 2007 and was driven by surprisingly consistent processes. Individual admixture was related to two phenotypic traits associated with fitness: size at spawning and age of juvenile emigration. Fish with higher non-native admixture were larger and tended to emigrate at a younger age-relationships that are expected to confer fitness advantages to hybrid individuals. However, strong selection against non-native admixture was evident across streams and cohorts (mean selection coefficient against genotypes with non-native alleles (s) = 0.60; s.e. = 0.10). Nevertheless, hybridization was promoted in both streams by the continuous immigration of individuals with high levels of non-native admixture from other hybrid source populations. Thus, antagonistic relationships between dispersal and selection are mediating invasive hybridization between these fish, emphasizing that data on dispersal and natural selection are needed to fully understand the dynamics of introgression between native and non-native species.
© 2014 The Author(s) Published by the Royal Society. All rights reserved.
Experience from case studies of biological invasions in aquatic ecosystems has motivated a set of proposed empirical ``rules'' for understanding patterns of invasion and impacts on native species. Further evidence is needed to better understand these patterns, and perhaps contribute to a useful predictive theory of invasions. We reviewed the case of brook trout (Salvelinus fontinalis) invasions in the western United States and their impacts on native cutthroat trout (Oncorhynchus clarki). Unlike many biological invasions, a considerable body of empirical research on brook trout and cutthroat trout is available. We reviewed life histories of each species, brook trout invasions, their impacts on cutthroat trout, and patterns and causes of segregation between brook trout and cutthroat trout. We considered four stages of the invasion process: transport, establishment, spread, and impacts to native species. Most of the research we found focused on impacts. Interspecific interactions, especially competition, were commonly investigated and cited as impacts of brook trout. In many cases it is not clear if brook trout invasions have a measurable impact. Studies of species distributions in the field and a variety of experiments suggest invasion success of brook trout is associated with environmental factors, including temperature, landscape structure, habitat size, stream flow, and human influences. Research on earlier stages of brook trout invasions (transport, establishment, and spread) is relatively limited, but has provided promising insights. Management alternatives for controlling brook trout invasions are limited, and actions to control brook trout focus on direct removal, which is variably successful and can have adverse effects on native species. The management applicability of research has been confounded by the complexity of the problem and by a focus on understanding processes at smaller scales, but not on predicting patterns at larger scales. In the short-term, an improved predictive understanding of brook trout invasions could prove to be most useful, even if processes are incompletely understood. A stronger connection between research and management is needed to identify more effective alternatives for controlling brook trout invasions and for identifying management priorities.
We explored potential negative effects of exotic brown trout (Salmo trutta) on native sculpin (Cottus sp.) on the Logan River, Utah, USA by (i) examining factors most strongly correlated with sculpin abundance (e.g., abiotic conditions or piscivory?), (ii) contrasting the extent of brown trout predation on sculpin with that by native cutthroat trout (Oncorhynchus clarkii utah) and (iii) estimating the number of sculpin consumed by brown trout along an elevational gradient using bioenergetics. Abundance of sculpin across reaches showed a strong (r ≥ 0.40) and significant (P < 0.05) correlation with physical variables describing width (positive) and gradient (negative), but not with abundance of piscivorous brown trout or cutthroat trout. In mainstem reaches containing sculpin, we found fish in 0% of age-1, 10% of age-2 and 33% of age-3 and older brown trout diets. Approximately 81% of fish consumed by brown trout were sculpin. Despite a similar length–gape relationship for native cutthroat trout, we found only two fish (one sculpin and one unknown) in the diets of native cutthroat trout similar in size to age-3 brown trout. Based on bioenergetics, we estimate that an average large (> 260 mm) brown trout consumes as many as 34 sculpin per year. Nevertheless, results suggest that sculpin abundance in this system is controlled by abiotic factors and not brown trout predation. Additional research is needed to better understand how piscivory influences brown trout invasion success, including in-stream experiments exploring trophic dynamics and interactions between brown trout and native prey under different environmental conditions.
Water temperature appears to play a key role in determining population persistence of westslope cutthroat trout Oncorhynchus clarkii lewisi, but specific thermal performance and survival criteria have not been defined. We used the acclimated chronic exposure laboratory method to determine upper thermal tolerances and growth optima of westslope cutthroat trout and rainbow trout O. mykiss, a potential nonnative competitor that occupies much of the former range of westslope cutthroat trout. Rainbow trout had a distinct survival advantage over westslope cutthroat trout at water temperatures above 20°C. The ultimate upper incipient lethal temperature of rainbow trout (24.3°C; 95% confidence interval [CI] = 24.0–24.7°C) was 4.7°C higher than that of westslope cutthroat trout (19.6°C; 95% CI = 19.1–19.9°C). In contrast, both species had similar growth rates and optimum growth temperatures (westslope cutthroat trout: 13.6°C; rainbow trout: 13.1°C) over the temperature range of 8–20°C, although rainbow trout grew over a wider range and at higher temperatures than did westslope cutthroat trout. The rainbow trout's higher upper temperature tolerance and greater growth capacity at warmer temperatures may account for the species' displacement of westslope cutthroat trout at lower elevations. Our results indicate that maximum daily temperatures near the optimum growth temperature of 13–15°C would ensure suitable thermal habitat for westslope cutthroat trout populations. The low upper temperature tolerance and optimum growth temperature of westslope cutthroat trout relative to those of other salmonids suggest that this subspecies may be particularly susceptible to stream temperature increases associated with global warming and anthropogenic habitat disturbance.
The Lahontan cutthroat trout Oncorhynchus clarki henshawi, an interior basin salmonid, is endemic to the hydrographic Lahontan Basin, which encompasses parts of northern Nevada, northeastern California, and southeastern Oregon. This subspecies is currently listed as threatened under the 1975 U.S. Endangered Species Act. Landscape- and population-level research suggests that this subspecies has survived in a desert environment by living in large, interconnected stream and/or stream-and-lake systems that support a metapopulation dynamic. Threats to the subspecies include habitat fragmentation as well as competition and hybridization with nonnative salmonids. Hybridization with the closely related rainbow trout O. mykiss compromises rangewide recovery efforts by increasing the risk of introgression and subsequent loss of pure populations in restored population networks. Here we use a suite of highly variable genetic markers (microsatellites, simple sequence repeats, and arbitrarily amplified regions) to assess levels of hybridization between Lahontan cutthroat trout and rainbow trout in the McDermitt Creek stream system located in the northwestern Lahontan Basin. The McDermitt Creek stream system is the only potentially networked stream system in the Quinn River/Black Rock Desert distinct population segment (DPS). Three populations (Sage Creek, Line Canyon Creek, and Indian Creek) targeted for reconnection were assessed for hybridization status. The Line Canyon Creek population was the only pure Lahontan cutthroat trout population. The Sage Creek and Indian Creek populations had varying levels of introgression. Fixation index (FST) results showed significant genetic differentiation among Quinn River, Humboldt River, and Western Basin DPS populations (pairwise FST estimates ≥ 0.3, P = 0.00064). Due to the limited number of pure populations in the Quinn River/Black Rock Desert DPS and their genetic distinctiveness, preservation of the populations is warranted, despite limited introgression.
In the western United States, the ability of non-native lake trout (Salvelinus namaycush) to attain large sizes, > 18 kg under favorable conditions, fueled the popularity of lake trout fisheries. In the past, restrictive regulations were adopted to increase lake trout abundance and produce trophy specimens. More recently, lake trout have become increasingly problematic because they prey upon and potentially compete with native and sport fishes. We review the experiences of agencies in seven western states which are considering or implementing strategies to address lake trout impacts despite management difficulties due to mixed public perception about lake trout's complex interactions with native or introduced fauna. Special regulations protecting lake trout have often been liberalized or rescinded to encourage their harvest and reduce their negative effects. More intensive methods to control or reduce lake trout abundance include promoting or requiring lake trout harvest, commercial-scale netting, disrupting spawning, and stocking sterile lake trout.
We describe the historical and current distributions and genetic status of westslope cutthroat trout Oncorhynchus clarkii lewisii (WCT) throughout its range in the western United States using data and expert opinion provided by fish managers. Westslope cutthroat trout historically occupied 90,800 km and currently occupy 54,600 km; however, these are probably underestimates due to the large-scale (1:100,000) mapping we used. Genetic analyses found no evidence of genetic introgression in 768 samples (58% of samples tested), but the numbers of individuals tested per sample were variable and sample sites were not randomly selected. Approximately 42% of the stream length occupied by WCT is protected by stringent land use restrictions in national parks (2%), wilderness areas (19%), and roadless areas (21%). A total of 563 WCT populations (39,355 km) are being managed as “conservation populations,” and while most (457, or 81%) conservation populations were relatively small, isolated populations, large and interconnected metapopulations occupied much more stream length (34,820 km, or 88%). While conservation populations were distributed throughout the historical range (occupying 67 of 70 historically occupied basins), they were much denser at the core than at the fringes. From the information provided we determined that conserving isolated populations (for their genetic integrity and isolation from nonnative competitors and disease) and metapopulations (for their diverse life histories and resistance to demographic extinction) is reasonable. We conclude that while the distribution of WCT has declined dramatically from historical levels, as a subspecies WCT are not currently at imminent risk of extinction because (1) they are still widely distributed, especially in areas protected by stringent land use restrictions; (2) many populations are isolated by physical barriers from invasion by nonnative fish and disease; and (3) the active conservation of many populations is occurring.
Several nonlethal methods have been developed to determine the stomach contents of fish, including gastroscopes, tubes, stomach suction, stomach flushing, emetics, forceps, and chronic fistulas. By reviewing the literature on this subject, we found that the effectiveness (ability to remove all stomach contents) of the different methods depends on size, age, species of fish, and the size of the food items in the stomach. Overall, various methods of stomach flushing were the most effective method of recovering stomach items from a variety of fishes. Mechanized pressure appeared to be the most efficient method of stomach flushing for most large fishes. The use of syringes allowed stomach flushing to be performed on most young and small fishes. The use of tubes and stomach suctions, much simpler and less expensive methods than stomach flushing, were nearly as effective for some fishes such as black bass (Micropterus spp.) and salmonids.
‘Successful’ introduced species are often thought to cause declines or extinctions of native species through competitive superiority. In western North America, introduced rainbow trout, Oncorhynchus mykiss, have completely replaced many native cutthroat trout, Oncorhynchus clarkii, populations; however, few studies have identified the mechanisms that may allow rainbow trout to outcompete cutthroat trout. We raised Yellowstone cutthroat trout, Oncorhynchus clarkii bouvieri, rainbow trout, and their first generation hybrids in a common environment and conducted pairwise contests to test for differences in aggression, ability to defend a feeding station, and amount of food captured between these species and their hybrids. We did not detect a difference in number of aggressive acts conducted between cutthroat, rainbow and hybrid trout; however, cutthroat trout had the lowest success in occupying the feeding station and captured a lower proportion of food than rainbow and hybrid trout. Furthermore, hybrid crosses and rainbow trout had highest success at holding the feeding station and capturing food items when competing against cutthroat trout. Our study suggests that juvenile Yellowstone cutthroat trout are less successful at maintaining profitable feeding territories and capturing food items when competing against rainbow trout and first generation hybrids.
Population declines are often caused by multiple factors, including anthropogenic ones that can be mitigated or reversed to enhance population recovery. We used a size-classified matrix population model to examine multiple anthropogenic effects on a population and determine which factors are most (or least) important to population dynamics. We modeled brook trout (Salvelinus fontinalis) in southern Appalachian mountain streams responding to multiple anthropogenic effects including the introduction of an exotic salmonid species (rainbow trout, Oncorhynchus mykiss), a decrease in pH (through acidic deposition), an increase in siltation (from roadbuilding and logging), and an increase in fishing pressure. Potential brook trout responses to rainbow trout include a decrease in survival rate of small fish, a change in density dependence in survival of small fish, and a decrease in growth rates of all sizes. When we included these responses in the population model, we found that population size tended to decrease with an increase in small-fish growth rate (producing a population with fewer, but larger, fish). In addition, changes in patterns of density-dependent survival also had a strong impact on both population size and size structure. Brook trout respond to decreases in pH with decreased growth rate in all size classes, decreased survival rates of small fish, and decreased egg-to-larva survival rates. This combination of effects, at magnitudes documented in laboratory experiments. had severe negative impacts on the modeled population. If siltation effects were severe, the extreme increase in egg-to-larva mortality could have strong negative effects on the population. However, even very strong increases in large fish mortality associated with sport harvesting were not likely to cause a local extinction. In all of these cases, the interaction of drastic changes in population size structure with randomly occurring floods or droughts may lead to even stronger negative impacts than those predicted from the deterministic model. Because these fish can reproduce at a small size, negative impacts on survival of the largest fish were not detrimental to the persistence of the population. Because survival of small juveniles is density dependent, even moderate decreases in survival in this stage had little effect on the ultimate population size. In general, a brook trout population will respond most negatively to factors that decrease survival of large juveniles and small adults, and growth rates of small juveniles.
Temperature-mediated competition (i.e., dominance shifts between species depending on temperature) may explain the segregation of salmonid species along altitudinal stream gradients. We evaluated this hypothesis for exotic brown trout (Salmo trutta) and native Bonneville cutthroat trout (Oncorhynchus clarkii utah) by rearing them in experi- mental sympatry and allopatry using enclosures constructed at six sites spaced along a 45-km segment of a mountain stream. For both species, we compared condition and growth between allopatric and sympatric treatment groups. We found that brown trout negatively affected cutthroat trout performance, whereas cutthroat trout failed to impart an effect in the reverse direction, regardless of temperature. Thus, we documented asymmetric competition between these species but found little evidence indicating that its outcome was influenced by temperature. Brown trout - cutthroat trout seg- regation is therefore unlikely to be due to temperature-mediated competition. Instead, brown trout may have displaced cutthroat trout from downstream areas through competition or other mechanisms, while abiotic factors preclude their (brown trout) invasion of upper elevations. Given the magnitude of effect observed in our study, we recommend that brown trout receive greater consideration in cutthroat trout conservation.
Recent and projected climate warming trends have prompted interest in impacts on coldwater fishes. We examined the role of climate (temperature and flow regime) relative to geomorphology and land use in determining the observed distributions of three trout species in the interior Columbia River Basin, USA. We considered two native species, cutthroat trout (Oncorhynchus clarkii) and bull trout (Salvelinus confluentus), as well as nonnative brook trout (Salvelinus fontinalis). We also examined the response of the native species to the presence of brook trout. Analyses were conducted using multilevel logistic regression applied to a geographically broad database of 4165 fish surveys. The results indicated that bull trout distributions were strongly related to climatic factors, and more weakly related to the presence of brook trout and geomorphic variables. Cutthroat trout distributions were weakly related to climate but strongly related to the presence of brook trout. Brook trout distributions were related to both climate and geomorphic variables, including proximity to unconfined valley bottoms. We conclude that brook trout and bull trout are likely to be adversely affected by climate warming, whereas cutthroat trout may be less sensitive. The results illustrate the importance of considering species interactions and flow regime alongside temperature in understanding climate effects on fish.
The factors controlling the effects of introduced trout on native galaxiid fishes in New Zealand streams were investigated by quantitative and qualitative electrofishing. Habitat assessments indicated that bed stability was closely related to total fish biomass. Exotic trout were not present at the most unstable sites but inhabited most medium to large streams with stable beds. Galaxiids (Galaxias vulgaris, Galaxias brevipinnis, and Galaxias paucispondylus) occurred at sites that spanned almost the entire range of habitat conditions and co-occurred with trout more frequently at intermediate levels of bed stability. However, galaxiids were absent from all sites with large (>150-mm fork length (FL)) brown (Salmo trutta) and rainbow (Oncorhynchus mykiss) trout. These streams tended to be larger, with more stable beds. In a field tank experiment, large brown trout consumed Canterbury galaxias (G. vulgaris), ranging in size from 48- to 94-mm FL, at a much higher rate than did small trout. Their predation of galaxiids appeared not to be size selective, so there was no size refuge for galaxiids from predation by large trout. These results indicate that predation by large trout has likely eliminated small-bodied galaxiids from many streams but that trout impact is limited by the availability of habitats suitable for large individuals.
Understanding the vulnerability of aquatic species and habitats under climate change is critical for conservation and management of freshwater systems. Climate warming is predicted to increase water temperatures in freshwater ecosystems worldwide, yet few studies have developed spatially explicit modelling tools for understanding the potential impacts. We parameterized a nonspatial model, a spatial flow-routed model, and a spatial hierarchical model to predict August stream temperatures (22-m resolution) throughout the Flathead River Basin, USA and Canada. Model comparisons showed that the spatial models performed significantly better than the nonspatial model, explaining the spatial autocorrelation found between sites. The spatial hierarchical model explained 82% of the variation in summer mean (August) stream temperatures and was used to estimate thermal regimes for threatened bull trout (Salvelinus confluentus) habitats, one of the most thermally sensitive coldwater species in western North America. The model estimated summer thermal regimes of spawning and rearing habitats at
We tested the elevation refuge hypothesis that colder temperatures impart a competitive advantage to bull trout Salvelinus confluentus and thus account for increased biotic resistance to invasion and displacement by brook trout S. fontinalis in headwater streams. Growth, survival, and behavior were compared in allopatry and sympatry at temperatures of 8-20°C in the laboratory. In allopatry, age-0 bull trout and brook trout grew at similar rates at temperatures of 8.0-14.3°C, but brook trout grew significantly faster at higher temperatures. In sympatry, bull trout grew significantly less than brook trout at all test temperatures, with growth differences increasing linearly with increased temperature. Age-1 brook trout had significantly higher feeding and aggression rates than did similar-sized bull trout at 8°C and 16°C. The modeled growth of age-0 bull trout and brook trout based on tributary temperature data from a high-elevation site (mean summer temperature, 10°C) and a low-elevation site (14°C) was similar for both species in allopatry. However, brook trout achieved much greater size than bull trout in sympatry, particularly at the warm site, where the predicted size of brook trout was 21.7 mm (23%) greater in length and 4.9 g (60%) greater in weight. Brook trout therefore had a marked size and growth advantage over bull trout at warm temperatures, but bull trout do not appear to gain a similar advantage over brook trout at low temperatures. Thus, factors in addition to water temperature are relevant to protecting remaining bull trout populations from displacement by brook trout in headwater streams.
Native salmonid fishes often face simultaneous threats from habitat fragmentation and invasion by nonnative trout species. Unfortunately, management actions to address one may create or exacerbate the other. A consistent decision process would include a systematic analysis of when and where intentional use or removal of barriers is the most appropriate action. We developed a Bayesian belief network as a tool for such analyses. We focused on native westslope cutthroat trout (Oncorhynchus clarkii lewisi) and nonnative brook trout (Salvelinus fontinalis) and considered the environmental factors influencing both species, their potential interactions, and the effects of isolation on the persistence of local cutthroat trout populations. The trade-offs between isolation and invasion were strongly influenced by size and habitat quality of the stream network to be isolated and existing demographic linkages within and among populations. An application of the model in several sites in western Montana (USA) showed the process could help clarify management objectives and options and prioritize conservation actions among streams. The approach can also facilitate communication among parties concerned with native salmonids, nonnative fish invasions, barriers and intentional isolation, and management of the associated habitats and populations.
Predation has a fundamental role in aquatic ecosystems, but the relative importance of factors governing prey selection by predators remains controversial. In this study, we contrast five lakes of a subarctic watershed to explore how prey community characteristics affect prey selection and growth rate of the common top predator, brown trout (Salmo trutta). The lakes constitute a distinct gradient of different coregonid prey fish, ranging from monomorphic common whitefish (Coregonus lavaretus) to polymorphic whitefish co-occurring with vendace (Coregonus albula). The brown trout was a morph-species- and size-specific pelagic predator, selecting the small-sized, pelagic whitefish morph or vendace over the benthic whitefish morphs. In all lakes, the average prey size increased with predator size, but small-sized prey were also included in the diet of large predators. The selection of small-sized, pelagic prey fish appeared to be a favourable for aging strategy for the brown trout, yielding higher growth rates and an earlier ontogenetic shift to piscivory. The findings emphasize that piscivory appear to be shaped by the diversity, size-structure, and abundance of available prey in a given community.
We sampled 19 isolated headwater populations of westslope cutthroat trout Oncorhynchus clarki lewisi in Montana to provide estimates of fecundity, longevity, sex ratio, and age at sexual maturity. Fecundity was estimated for 31 fish collected from two streams in the upper Missouri River drainage. Females smaller than 149 mm fork length (FL) were generally immature and their fecundities could not be estimated. Mean fecundities (SD) were 227 eggs (41.1) for 150–174-mm fish, 346 eggs (85.6) for 175–199-mm fish, and 459 eggs (150.8) for 200-mm and larger fish. A linear regression model (two stream samples combined) to predict fecundity (E) from fork length was developed (E = –494.9 + 4.4sFL; r 2 = 0.51, P < 0.001) for westslope cutthroat trout in the upper Missouri River drainage. Regression slopes of fecundity against fish length differed significantly (P < 0.01) between these and some of the previously studied populations. Steeper slopes were associated with lacustrine-adfluvial populations. The average sex ratio was 1.3 males per female across all sampled streams. Males began to mature sexually at age 2 and all were mature by age 4. Some females (27%) were sexually mature at age 3 and most of them (93%) were mature by age 5. Length was a better predictor of sexual maturity than age. Males matured at 110–160 mm and females at 150–180 mm FL. The maximum estimated age was 8 years based on otoliths from 475 fish collected from our 19 study streams and 14 additional streams.
Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri were historically distributed in the Yellowstone River drainage (Montana and Wyoming) and the Snake River drainage (Wyoming, Idaho, Utah, Nevada, and probably Washington). Individual populations evolved distinct life history characteristics in response to the diverse environments in which they were isolated after the last glaciation. Anthropogenic activities have resulted in a substantial decline (42% of the historical range is currently occupied; 28% is occupied by core [genetically unaltered] populations), but the number of extant populations, especially in headwater streams, has precluded listing of this taxon under the Endangered Species Act. Primary threats to persistence of Yellowstone cutthroat trout include (1) invasive species, resulting in hybridization, predation, disease, and interspecific competition; (2) habitat degradation from human activities such as agricultural practices, water diversions, grazing, dam construction, mineral extraction, grazing, timber harvest, and road construction; and (3) climate change, including an escalating risk of drought, wildfire, winter flooding, and rising temperatures. Extirpation of individual populations or assemblages has led to increasing isolation and fragmentation of remaining groups, which in turn raises susceptibility to the demographic influences of disturbance (both human and stochastic) and genetic factors. Primary conservation strategies include (1) preventing risks associated with invasive species by isolating populations of Yellowstone cutthroat trout and (2) connecting occupied habitats (where possible) to preserve metapopulation function and the expression of multiple life histories. Because persistence of isolated populations may be greater in the short term, current management is focused on isolating individual populations and restoring habitats; however, this approach implies that humans will act as dispersal agents if a population is extirpated because of stochastic events.Received August 19, 2010; accepted April 28, 2011
There is a long history of introduction of non-native fishes in fresh waters and the introduction rate has accelerated greatly over time. Although not all introduced fishes have appreciable effects on their new ecosystems, many exert significant ecological, evolutionary, and economic impacts. For researchers, managers, and policy makers interested in conserving freshwater diversity, understanding the magnitude and array of potential impacts of non-native fish species is of utmost importance. The present study provides an illustrative conspectus of the most recent literature reporting ecological impacts of non-native freshwater fishes from a wide range of species and geographic locations and concludes with a prospectus of needed areas of scientific inquiry. Both directly and indirectly, invasive fishes affect a wide range of native organisms from zooplankton to mammals across multiple levels of biological organizations ranging from the genome to the ecosystem. Although a great deal of knowledge has been recently accumulated, this body of knowledge dwarfs in comparison to what we still need to learn. Specifically, we cite the need for additional scientific inquiry to fill knowledge gaps that are principally caused by taxonomically, geographically, disciplinarily, and methodologically unbalanced approaches.
In western North America, nonnative trout invasions threaten the persistence of native cutthroat trout Oncorhynchus clarkii subspp. through competition, predation, and hybridization. While small-scale individual-level studies on nonnative–native fish interactions have helped elucidate mechanisms of impact, very few investigations have occurred at a scale consistent with that of species replacement. Thus, large-scale population-level studies are needed to understand the effects of nonnative fish as well as to more effectively conserve native fishes in the presence of nonnative fish. For this reason, we conducted an evaluation of the effects of nonnative brown trout Salmo trutta on the individual- and population-level performance of native cutthroat trout using a field-experimental approach. We studied local populations of cutthroat trout in the presence and absence of brown trout in long stream reaches and subsequently compared growth, condition, stable-isotope-based measures of dietary habits, movement, and survival between treatment groups. We found clear evidence for reduced cutthroat trout growth owing to the presence of brown trout. Additionally, we observed differences in cutthroat trout dietary habits based on δC signatures resulting from our experimental manipulation of brown trout abundance. Finally, brown trout suppressed cutthroat trout movement but had no apparent effect on their survival. Considering previous studies, we concluded that the individual-level effects of brown trout on cutthroat trout are scale invariant. Higher-level impacts (i.e., on movement and survival), in contrast, appear to be more sensitive to the scale at which the investigation is conducted. Overall, our results indicate that brown trout can have strong negative effects on cutthroat trout performance and should therefore be more explicitly considered in native fish conservation plans.
Brown trout in Lake Femund migrated from the nursery streams mainly at 2 years old, but ranging between 1 and 8 years. Brown trout switched to piscivory from 3 years onwards, and a body length of 17·5 cm, according to back calculation from scales. Fast growers switched to piscivory at a younger age and smaller size than slow growers. The most slow-growing trout switched to fish feeding at 9 years old and a mean body length of 36 cm. The growth of the invertebrate feeders was almost rectilinear to c. 45 cm and 11 years of age. Switching to a fish diet induced increased growth rates. Age at sexual maturity increased with the age at which the fish became piscivorous. The invertebrate feeders matured at an age similar to that of the most fast growing piscivorous trout. The mortality rate of sub-adults and adults did not differ significantly between fish and invertebrate feeding trout. Longevity of piscivorous trout was estimated at 11 years and of invertebrate feeders at 10 years.
Effective conservation of Cutthroat Trout Oncorhynchus clarkii lineages native to the Rocky Mountains will require estimating effects of multiple stressors and directing management toward the most important ones. Recent analyses have focused on the direct and indirect effects of a changing climate on contemporary ranges, which are much reduced from historic ranges owing to past habitat loss and nonnative trout invasions. However, nonnative trout continue to invade Cutthroat Trout populations in the southern Rocky Mountains. Despite management to isolate and protect these native populations, nonnatives still surmount barriers or are illegally stocked above them. We used data on the incidence of invasions by nonnative Brook Trout (BT) Salvelinus fontinalis and the rate of their invasion upstream to simulate effects on a set of 309 conservation populations of Colorado River Cutthroat Trout (CRCT) O. c. pleuriticus isolated in headwater stream fragments. A previously developed Bayesian network model w...
The Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri is native to the Rocky Mountains and has declined in abundance and distribution as a result of habitat degradation and introduced salmonid species. Many of its remaining strongholds are in headwater basins with minimal human disturbances. Understanding the life histories, vital rates, and behaviors of Yellowstone Cutthroat Trout within headwater stream networks remains limited yet is critical for effective management and conservation. We estimated annual relative growth in length and weight, annual survival rates, and movement patterns of Yellowstone Cutthroat Trout from three tributaries of Spread Creek, Wyoming, and two tributaries of Shields River, Montana, from 2011 through 2013 using PIT tag antennas within a mark–recapture framework. Mean annual growth rates varied among tributaries and size-classes, but were slow compared with populations of Yellowstone Cutthroat Trout from large, low-elevation streams. Survival rates were relatively high compared with those of other Cutthroat Trout subspecies, but we found an inverse relationship between survival and size, a pattern contrary to what has been reported for Cutthroat Trout in large streams. Mean annual survival rates ranged from 0.32 (SE = 0.04) to 0.68 (SE = 0.05) in the Spread Creek basin and from 0.30 (SE = 0.07) to 0.69 (SE = 0.10) in the Shields River basin. Downstream movements from tributaries were substantial, with as much as 26.5% of a tagging cohort leaving over the course of the study. Integrating our growth, survival, and movement results demonstrates the importance of considering strategies to enhance headwater stream habitats and highlights the importance of connectivity with larger stream networks.
Received February 17, 2016; accepted June 18, 2016
Estimates of age and lengths at specific ages of Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri Richardson) were made using otoliths and scales. Fish were sampled from 17 high-elevation streams in the Greybull River drainage, Wyoming. Variation in estimates of age within and among three readers were assessed using both structures. Variability among age estimates by individual readers was low for both structures. Estimates using otoliths were significantly less variable than were estimates based on scales both among readers and among estimates by individual readers. Otoliths were more accurate than scales for estimating the correct age of fish. Back-calculated estimates of fish lengths at given ages based on otoliths were significantly less than those based on scales, Hatchery fish grew faster than wild fish at younger ages. Overall, growth of wild fish was slower than in other areas where Yellowstone cutthroat trout are endemic. We predict that if otoliths were used instead of scales to assess age and growth of other trout species in high-elevation streams that similar differences in estimates based on the two structures would be observed.
Anders Halverson provides an exhaustively researched and grippingly rendered account of the rainbow trout and why it has become the most commonly stocked and controversial freshwater fish in the United States. Discovered in the remote waters of northern California, rainbow trout have been artificially propagated and distributed for more than 130 years by government officials eager to present Americans with an opportunity to get back to nature by going fishing. Proudly dubbed "an entirely synthetic fish" by fisheries managers, the rainbow trout has been introduced into every state and province in the United States and Canada and to every continent except Antarctica, often with devastating effects on the native fauna. Halverson examines the paradoxes and reveals a range of characters, from nineteenth-century boosters who believed rainbows could be the saviors of democracy to twenty-first-century biologists who now seek to eradicate them from waters around the globe. Ultimately, the story of the rainbow trout is the story of our relationship with the natural world-how it has changed and how it startlingly has not.
This chapter gives results from some illustrative exploration of the performance of information-theoretic criteria for model selection and methods to quantify precision when there is model selection uncertainty. The methods given in Chapter 4 are illustrated and additional insights are provided based on simulation and real data. Section 5.2 utilizes a chain binomial survival model for some Monte Carlo evaluation of unconditional sampling variance estimation, confidence intervals, and model averaging. For this simulation the generating process is known and can be of relatively high dimension. The generating model and the models used for data analysis in this chain binomial simulation are easy to understand and have no nuisance parameters. We give some comparisons of AIC versus BIC selection and use achieved confidence interval coverage as an integrating metric to judge the success of various approaches to inference.
1. A growth model for brown trout, developed almost 20 years ago, has been used to investigate growth potential in at least 40 populations over a wide geographical range. The chief disadvantages of the model are: it is based on growth data for only 55 hatchery trout kept in tanks without strict control of temperature and oxygen, it is not continuous and is restricted to the range 3.8-19.5-degrees-C, it requires six parameters and only one of these can be interpreted biologically. 2. For the new model, growth data were obtained for an additional 130 trout bred from wild parents and kept in tanks at five constant temperatures (range +/- 0.1 or 0.2-degrees-C) and 100% oxygen saturation. The new model is continuous over the range 3.8-21.7-degrees-C and has five parameters, all of which can be interpreted in biological terms. It was fitted to growth data for individual fish and was an excellent fit (P < 0.001, R2 > 0.99) to the data for the 55 trout of the original experiment, the 130 trout of the new experiment and both experiments combined. The procedure for applying the model to field data is critically examined and a suitable test for maximum growth potential is described. The model ceases to be robust when mean temperatures are estimated over periods of more than 3 months. 3. Although parameter estimates for the new model are similar for the original and new experiments, they are significantly different. An iterative exercise, varying common and different parameters, showed this to be the result of slight differences between two parameters; the optimum temperature for growth and the growth rate of a 1-g fish at this temperature. Possible reasons for this are discussed and it is concluded that these differences have a negligible effect on values predicted from the model.
The occupation of adjacent, nonoverlapping positions along environmental gradients by closely related and ecologically similar species has drawn considerable attention from many ecologists over the past decades. Condition-specific competition, wherein competitive superiority varies with the abiotic environmental gradient, has been proposed as the major structuring force behind such distributions. However, few studies have elucidated the underlying mechanisms, such as behavioral and demographic processes. We conducted laboratory experiments to examine the effects of temperature on interspecific competition between two stream salmonid fishes, Salvelinus malma and S. leucomaenis. The two species have a largely allopatric altitudinal distribution on Hokkaido Island, Japan, proposed to be the result of temperature-mediated competition. We tested predictions that at a higher temperature (12°C), S. leucomaenis would dominate over S. malma in aggressive interactions, foraging performance, growth, and survival, but become subordinate at a lower temperature (6°C). Indeed, S. leucomaenis initiated a greater number of aggressive acts, attained greater food intake and greater growth, and finally excluded S. malma at the higher temperature. Although the two species initiated a similar number of aggressive acts and foraged equally well at the lower temperature, S. leucomaenis achieved a higher growth rate than S. malma; however, the latter eventually became numerically dominant. Clear competitive release in allopatry occurred for S. malma only at the higher temperature, providing direct evidence of condition-specific asymmetric competition. The lower distribution boundary of S. malma in Hokkaido streams may therefore be determined by temperature-mediated condition-specific competition. However, mechanisms determining the upper distribution boundary of S. leucomaenis could not be fully explained by the competitive results at lower temperature, but required an understanding of how effects of competition interacted with species-specific physiological traits. Thus, species distributions along an environmental gradient cannot be solely explained by a simple model of condition-specific competition without considering mechanistic linkages among behavioral and physiological responses to the environment, resource use, and demographic processes.
Objective, empirical measures of overlap between samples of items distributed proportionally into various qualitative categories are presented and reviewed. These indices of overlap, derived from either probability or information theory, should prove useful to the ecologist in comparative studies of diet, habitat preference, seasonal patterns of abundance, faunal lists, or similar data.
A model is developed to predict potential net energy gain for salmonids in streams from characteristics of water velocity and invertebrate drift. Potential net energy gain, or potential profit, is calculated for individuals of three species of juvenile salmonids in a laboratory stream aquarium, based on the energy available from drift minus the cost of swimming to maintain position. The Michaelis–Menten or Monod model is used to describe the relationship between potential profit and specific growth rate. Potential profit was a better predictor of specific growth rate for coho salmon (Oncorhynchus kisutch) than for brook trout (Salvelinus fontinalis) or brown trout (Salmo trutta). Coho salmon always achieved higher specific growth rates than either brook trout or brown trout in concurrent experiments, and maintained growth to lower resource thresholds. In each experiment, fish established intraspecific hierarchies and dominant fish held positions affording maximum potential profit. The use of potential profit as an optimal foraging model was tested by predicting the potential for net energy gain at coho salmon positions from the overall pattern of flow and invertebrate drift in the stream aquarium, and ranking these positions from highest to lowest potential profit. This predicted ranking was nearly identical to the rank observed in the linear dominance hierarchy. The results of experiments confirm ideas of other investigators about mechanisms of microhabitat selection by stream salmonids.
A review of 17 controlled experiments of interspecific competition between juveniles of Atlantic salmon (Salmo salar) and other fishes revealed relatively little evidence to judge competitive effects at any scale. More than half were unreplicated and so inadequate to test either the existence or relative strength of interspecific competition. Most replicated experiments used one of two designs appropriate to address questions of interest, such as whether nonnative species affect Atlantic salmon via competition or whether interspecific competition from coevolved salmonids is greater than intraspecific competition. Replicated experiments spanned a broad range of spatial and temporal scales, and one well-designed field experiment yielded the strongest inference at useful scales. Nonnative salmonids being introduced worldwide into Atlantic salmon waters have the potential to invade, so experiments testing their effects are most urgently needed. Overall, juvenile coho salmon (Oncorhynchus kisutch) are suspected to have the greatest effect, partly due to their inherent size advantage. The potential for complex interactions or indirect effects to modify effects of nonnative species is completely unknown but may be important and needs investigation.
To examine the effect of introduced brown trout Salmo trutta on populations of native Galaxias truttaceus (Galaxiidae), known locally as spotted galaxias, population abundance models based on the habitat use patterns of G. truttaceus were used to compare streams with and without brown trout. In selected streams in southeast Tasmania, habitat use by G. truttaceus was examined with respect to four principal components extracted from eight habitat variables. Different size-classes of G. truttaceus displayed varying nonrandom patterns of habitat use, shifting from shallow, open habitats to deep, cover-rich habitats with increasing size. All size-classes preferred slow-flowing sections to fast-flowing sections. Population abundance models were constructed for three size-classes of G. truttaceus, and given the hydrologically variable nature of the streams studied, all of the models were reasonably successful in explaining variation. The application of the models to streams containing brown trout indicated that the presence of brown trout was more important than habitat characteristics in determining the abundance of G. truttaceus. In streams with brown trout, the density of each size-class of G. truttaceus was substantially less than that expected on the basis of habitat characteristics. The study provides strong evidence that brown trout adversely affect populations of G. truttaceus, because habitat differences were quantitatively accounted for when streams with and without brown trout were compared.
1. The chief objective of the present study was to develop a functional model for the daily change in the total energy content of a brown trout, Salmo trutta , (equivalent to growth when positive) in relation to the difference between energy intake (energy content of food) and energy losses (metabolism + losses in faeces and excretory products). Energy budgets for individual fish were obtained in earlier experiments with 210 hatchery trout (live weight = 11–270 g) kept at fairly constant temperatures (mean values ranging from 3.6 to 20.4 °C), but without strict control of temperature or oxygen, and in later experiments, with 252 trout (1–300 g) bred from wild parents and kept at five constant temperatures (5, 10, 13, 15 and 18 °C) and 100% oxygen saturation. Each trout was fed a fixed ration of shrimps, Gammarus pulex , the ration level varying between zero and maximum.
2. Energy intake ( C IN , cal day ⁻⁻¹ ) was measured directly and expressed as a proportion ( p ) of the maximum energy intake ( C , cal day ⁻⁻¹ ), the latter being estimated from a model developed earlier. In a new model, energy losses ( C Q , cal day ⁻⁻¹ ) were expressed as a function of temperature, fish weight and ration level. This model was continuous over the 3.6–20.4 °C range, had twelve fitted parameters and was an excellent fit to the data for the 462 trout ( P < 0.001, R ² = 0.9970). In an extended model, the weight exponent for energy losses was not assumed equal to that for energy intake, the difference between the two exponents being very small, but significant, with a slight improvement in the fit of the model ( R ² increased to 0.9972).
3. The limits of model use were discussed. An example of its utility was to elucidate the complex relationships between both positive (growth) and negative daily changes in the total energy content of the trout, and temperature, fish size and variable energy intake. The model has raised several questions for future work, including the effect of increasing energy intake by a change of diet from invertebrates to fish or fish pellets, and a comparison of growth models based on weight or energy changes.
Expressed as percentages of total fresh body weight, lipids of brook trout Salvelinus fontinalis declined between October and April: reproductive males from 2·89 to 1·22%, reproductive females from 3·19 to 1·84%, and non-reproductive males and females from 2·75 to 2·08%. The absolute and proportional overwinter reduction in lipids among reproductive trout was more than twice that of non-reproductive trout, with reproductive males losing significantly more lipids than reproductive females. Larger reproductive individuals lost more lipids during winter, relative to body size, than smaller individuals, although such an effect was not evident among non-reproductive trout. The average overwinter reduction in lipids for reproductive males (58%), females (42%), and non-reproductive trout (24%) was negatively associated with mark-recapture estimates of overwinter survival probabilities of 0·27, 0·36, and 0·58, respectively, providing support for the hypothesis that energy is allocated to reproduction to the detriment of post-reproductive survival. Our emergent hypothesis that reproductive costs differ between sexes, and the life history consequences thereof, merit further study.